Heme-hemopexin-mediated induction of metallothionein gene expression

Hemopexin-mediated heme transport into mouse hepatoma (Hepa) cells and human promyelocytic (HL-60) cells stimulates the expression of heme oxygenase via transcriptional activation (Alam, J., and Smith, A. (1989) J. Biol. Chem. 264, 17637-17640). Incubation of both these cell types in serum-free medium containing heme-hemopexin is shown here also to increase the steady-state level of metallothionein (MT) mRNA in a time- and dose-dependent manner. Heme-hemopexin is a far more effective inducer (12-fold) of the MT isozyme 1 (MT-1) in Hepa cells than nonprotein-bound heme (4-fold). Apohemopexin has no effect on MT-1 expression, and incubation with heme-hemopexin of mouse L fibroblasts that lack hemopexin receptors does not affect MT-1 expression. Thus, an interaction between the heme-hemopexin complex and its receptor is necessary for increased accumulation of MT-1 transcripts. In vitro nuclear "run-on" analysis indicates that the heme-hemopexin-mediated accumulation of MT-1 mRNA is regulated primarily at the level of initiation of transcription. A highly labile protein is required for constitutive MT-1 gene expression and acts to repress transcription. Transcriptional activation by heme or metals may require decreased concentrations or inactivation of the repressor as well as an additional inducer-specific trans-acting factor. Inhibition of protein synthesis augments the heme-hemopexin-mediated accumulation of MT-1 mRNA. Activation of heme oxygenase (HO) gene transcription by heme requires the synthesis of one (or more) heme-inducible proteins that are labile or become labile upon cycloheximide-sensitive processing or activation. Our comparison of MT and HO points to significant differences in the mechanisms of gene regulation by heme. The concomitant regulation of gene expression of MT-1 and HO in response to heme-hemopexin appears to be a concerted adaptive response of the cells, mediated at the level of the plasma membrane hemopexin receptor, and may relate to the proposed role of MT as an intracellular antioxidant or to a need to sequester zinc which otherwise would compete with iron and occupy sites on regulatory proteins such as the iron-responsive elements.